DevicePolicyManager: Add key generation functionality.

This is the crux of the Verified Access feature implementation:
Adding the ability to generate KeyChain keys directly by the
secure hardware, rather than installing software-generated keys
into KeyChain.

Add generateKeyPair to the DevicePolicyManager, which delegates key
generation (via the DevicePolicyManagerService) to the KeyChainService.

Design highlights:
* The key generation is delegated via the DevicePolicyManagerService to
  check that only authorized callers request key generation in KeyChain.
* KeyChainService performs the actual key generation so it owns the key
  in Keystore outright.
* DevicePolicyManagerService then grants the calling app access to the
  Keystore key, so it can actually be used.
* Loading the public/private key pair, as well as attestation
  certificate chain, is done in the client code (DevicePolicyManager)
  to save parceling / unparceling those objects across process
  boundaries twice (for no good reason).

NOTE: The key attestation functionality (that includes Device ID) is
missing/untested. Will be added in a follow-up CL as this one is quite
big already.

HIGHLIGHT FOR REVIEWERS:
* API: New API in DevicePolicyManager.

Bug: 63388672
Test: cts-tradefed run commandAndExit cts-dev -a armeabi-v7a -m CtsDevicePolicyManagerTestCases -t com.android.cts.devicepolicy.DeviceOwnerTest#testKeyManagement -l DEBUG; adb shell am instrument 'android.security.tests/android.support.test.runner.AndroidJUnitRunner' (After building the KeystoreTests target and installing the apk)
Change-Id: I73762c9123f32a94d454ba4f8b533883b55c44cc